Plasma deposition refers to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Plasma deposition processes may also be used to apply coatings to various objects to improve or change the properties of the objects. For example, plasma deposition processes may be used to apply wear resistant coatings to machine tools, while other types of coatings may be used to increase the corrosion resistance of other items, such as bearings, turbine blades, etc, thereby enhancing their performance. In still other applications, plasma deposition may be used to apply coatings to various types of surfaces in the optics and glass industries.
In most plasma deposition processes the plasma is created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field. The electric field, which is typically created between two electrodes, ionizes the process gas and creates the plasma. In the case of a DC sputter deposition plasma process, the material (commonly referred to as the target) to be deposited on the object or substrate is connected as the negative electrode (i.e., cathode), whereas some other element, commonly the vacuum chamber itself, is connected as the positive electrode or anode. Ionized process gas atoms comprising the plasma are accelerated toward and ultimately impact the negatively charged cathode/target, dislodging or sputtering atoms from the target material. The sputtered atoms subsequently condense on various items in the chamber, including the substrate that is to be coated. The substrate is usually positioned with respect to the target so that a majority of the sputtered target atoms condense on the surface of the substrate.
Sputter deposition processes of the type described above are usually referred to as "non-reactive" processes in that the sputtered atoms deposited on the surface of the substrate do not react with other materials. However, sputter deposition processes have also been developed wherein the target material is sputtered in the presence of a reactive material (e.g., oxygen or nitrogen gas). Such "reactive" sputtering processes may be used to deposit a film comprising the sputtered target material and the reactive species. A wide variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, and TiO, can be deposited by reactive sputter deposition processes.
While reactive sputtering processes are known and have been used for years, they continue to be plagued by the periodic occurrence of electrical discharges or arcs within the vacuum chamber. Such electrical discharges or arcs can take on different forms depending on the characteristics of the sputtering apparatus and on the particular plasma process being used. For example, arcs may occur between the target material, which is typically connected as the cathode in a DC sputtering process, and the substrate itself, certainly causing a defect in the coating, if not ruining the substrate entirely. Alternatively, the arc may occur between the target and some other part of the vacuum chamber, in which case the deleterious effects of the arc are usually less severe, but nevertheless tend to degrade the overall quality of the coating. The arcs can be short lived, lasting only a few milliseconds or so, or may be continuous, again depending on the particular apparatus and process being used. While such arcing can occur in nearly every kind of plasma process, the tendency of such arcs to occur is much greater in reactive processes where the compound film being deposited is an electrical insulator, such as Al.sub.2 O.sub.3.
Several methods for preventing, or at least reducing the frequency of such arc discharges, rely on the selective control of the power supply used to place the charge on the electrodes. For example, one such method has been to simply turn-off the power supply as soon as an arc is detected, then turn it back on again once the arc has dissipated. While this method can effectively quench sustained arcs, the stored energy in most power supplies takes time to dissipate, increasing the response time, i.e., the time it takes to remove the electrical potential from the electrodes, to the point where such devices cannot effectively quench short duration arc events. Consequently, all that is really accomplished is a reduction in overall deposition rate, with little or no reduction in the adverse effects produced by the arc event itself.
Another control method has been to momentarily interrupt (i.e., disconnect) the power supply from the electrodes during the arc event. While the response time of this method is usually considerably faster, i.e., the voltage can be removed from the electrodes within a few milliseconds or so, it is difficult to dissipate the stored energy in the power supply. Consequently, such methods tend to stress the power supply or switching devices used to disconnect the power supply to the point of burn-out.
Another method of interrupting the voltage placed on the electrodes has been to use a tapped inductor connected in series between one terminal of the power supply and one of the electrodes. When an arc is detected, the center tap of the inductor is momentarily connected to the other terminal of the power supply. This has the effect of momentarily reversing the voltage on the electrodes. In certain cases, the magnitude of the reversed voltage charge is usually sufficient to quench the arc. Unfortunately, however, this method is not effective in suppressing arcs having impedances lower than the impedance of the switching network and center tap, which is a common occurrence. Consequently, the use of such center-tapped inductors has not proven to be a panacea.
While other devices exist and are being used with some degree of success, none are without their disadvantages. For example, many such other devices can only effectively suppress certain types of arc events or only arcs created under certain conditions. Other devices may have more effective arc suppression characteristics, but are usually plagued with complex electronic circuits and devices, which may be expensive to produce and/or prone to failure.
Consequently, a need exists for a method and apparatus for preventing and/or suppressing arc events in plasma processes and under various operating conditions. Such a method and apparatus should allow for the effective suppression of arcs under a wide range of conditions, but without the need to resort to expensive or complex circuit elements. Additional advantages could be realized if such a device could be used in conjunction with conventional power supplies.